A thermal processing chamber, as used herein, refers to a device that uses light energy to heat objects, such as semiconductor wafers. Such devices typically include a substrate holder for holding a semiconductor wafer and a light source that emits light energy for heating the wafer. For monitoring the temperature of the semiconductor wafer during heat treatment, thermal processing chambers also typically include radiation sensing devices, such as pyrometers, that sense the radiation being emitted by the semiconductor wafer at a selected wavelength. By sensing the thermal radiation being emitted by the wafer, the temperature of the wafer can be calculated with reasonable accuracy.
One major problem in the design of rapid thermal processing chambers having an optical temperature measurement system, however, has been the ability to prevent unwanted light radiated by the heater lamps from being detected by the pyrometric instrumentation. Should unwanted light not being emitted by the semiconductor wafer be detected by the pyrometer, the calculated temperature of the wafer may unreasonably deviate from the actual or true temperature of the wafer.
In the past, various methods have been used to prevent unwanted thermal radiation from being detected by the pyrometer. For instance, physical barriers have been used before to isolate and prevent unwanted light being emitted by the heater lamps from coming into contact with the pyrometer. Physical barriers have been especially used in rapid thermal processing chambers in which the heater lamps are positioned on one side of the semiconductor wafer and the pyrometer is positioned on the opposite side of the wafer.
Besides physical barriers, spectral filters have also been used to limit the amount of light interference detected by the pyrometers. A spectral filter refers to a device that filters the light being emitted by the heater lamps prior to the light entering the rapid thermal processing chamber. For instance, spectral filters can operate by removing light being emitted by the heater lamps at the wavelength at which the pyrometer operates. Preferably, spectral filters absorb unwanted thermal radiation while at the same time being transparent to the thermal radiation being emitted by the heater lamps that is necessary to heat the semiconductor wafer.
One type of spectral filter that has been used in the past by the assignee of the present invention is a window made from fused silica. Fused silica glass is transparent to most light energy but is known to have several strong absorbing regions that are maximized at wavelengths of about 2.7 microns, 4.5 microns and at wavelengths equal to and greater than 5 microns. The light absorbing bands at 4.5 microns and at 5 microns and greater are typically common and present in most grades of silica. The light absorbing band at 2.7 microns, however, is caused by the presence of "water" in the form of hydroxide ions contained within the particular type of silica. Specifically, when it is desirable to filter light at a wavelength of 2.7 microns using silica, the silica should contain hydroxide ions in a concentration of at least about 0.1% by weight.
Because silica glass can effectively absorb light at wavelengths of 2.7, 4.5 and greater than 5 microns and is substantially transparent to all other wavelengths of light energy, silica glass makes an effective spectral filter when the pyrometer contained within the thermal processing chamber is configured to sense thermal radiation at one of the above wavelengths.
Although spectral filters can be very efficient in filtering unwanted light emitted by heater lamps in a rapid thermal processing chamber, the spectral filters can increase in temperature during operation of the chamber and interfere with the heat treating wafer process. For instance, it is possible for spectral filters to gradually heat up due to the absorption of light being emitted by the heater lamps and, more particularly, due to absorbing thermal radiation being emitted by the hot semiconductor wafer. When a spectral filter substantially increases in temperature, the spectral filter can adversely interfere with the ability of the thermal processing chamber to uniformly heat subsequent semiconductor wafers.
Most thermal processing chambers process semiconductor wafers one at a time. Ideally, it is highly desirable to exchange and process wafers at a high rate of speed. When doing so, problems occur when a spectral filter is not allowed sufficient time to cool down to lower temperatures when loading a new semiconductor wafer into the chamber. Thus, a need exists for an apparatus and process for keeping a spectral filter from substantially increasing in temperature during the operation of a thermal processing chamber.
In the past, it has been proposed to use cooling fluids in order to cool spectral filters during operation of a rapid thermal processing chamber. For instance, in U.S. Pat. Nos. 4,550,684 and 4,680,477 both to Mahawili, a cooled optical window for semiconductor wafer heating is disclosed. Specifically, both of the above patents disclose using two parallel plates positioned within a thermal processing chamber. In order to prevent the plates from heating up, a fluid, such as water, air or the like, is circulated between the plates.
Even though the prior art has proposed various configurations, however, further improvements in the ability to prevent the heater lamps and spectral filters from interfering with the heat treating process are needed. As speed and product specifications become more demanding, the ability to heat treat semiconductor wafers according to specific temperature regimes and to precisely monitor the temperature of the wafer are becoming extremely crucial. In this regard, the present invention is directed to further improvements in thermal processing chambers for heat treating semiconductor wafers. As will be made apparent from the following description, various features, aspects, and advantages of the present invention remain absent from the prior art.